Influence of rearing temperature during the larval and nursery periods on growth and sex differentiation in two Mediterranean strains of Dicentrarchus labrax

European sea bass Dicentrarchus labrax of the north‐western (NW) and south‐eastern (SE) Mediterranean Sea strains were exposed to different temperatures (13, 17 or 21° C) during the larval rearing (11–51 days post hatching, dph) or nursery periods (55–95 dph), in order to examine the effects of temperature on sex differentiation and subsequent growth during the first year of life. Higher growth was observed during exposure to higher temperatures, but fish of the NW strain exposed to 13 or 17° C during larval rearing exhibited compensatory growth once exposure to the lower temperatures finished, and as a result their final size at 300 dph was similar or greater to the group exposed to 21° C. Fish exposed to 17° C during the nursery period also had similar size to fish exposed to 21° C after 300 days of rearing, but the fish exposed to 13° C remained significantly smaller (ANOVA, n  = 55–100, P  < 0·05). There were significant differences in the sex ratio among the fish exposed to different temperatures during the two periods of rearing, with high temperature (21° C) resulting in a significantly higher percentage of males in the population, both in the NW (ANOVA, n  = 2, P  < 0·04) and SE populations (ANOVA, n  = 2, P  < 0·01). The masculinization effect of high temperature was significantly stronger during the larval rearing stage, both in the NW (ANOVA, n  = 2, P  < 0·005) and SE populations (ANOVA, n  = 2, P  < 0·01). None of the temperature manipulations could produce 100% females, suggesting that there is a part of the genetic component in sex differentiation which is not labile to environmental influence.